This invention relates to automatic frequency control circuits for television receivers operating on the superheterodyne principle. Such television receivers have tuners capable of receiving transmitted television signals, heterodyning them with locally generated oscillator signals and producing constant intermediate frequency (IF) signals for processing. This heterodyning or frequency transposition process is well known.
Each government grants frequency allocations and establishes signal standards for all television transmitters operating within its jurisdiction. While the present invention is useful in any system with signal carriers spaced a predetermined distance apart and receivers generating fixed IF frequencies, its use in conjunction with the system promulgated by the Federal Communications Commission (FCC) in the United States will be discussed.
According to FCC regulations, broadcasters must maintain a frequency spacing of 4.5 MHz between the picture and sound carriers. Thus, while the frequencies of the carriers differ with different channel allocations (sound carrier is higher in frequency than the picture carrier), their frequency separation is constant. In the receiver, a local oscillator is adjusted (generally to a frequency higher than the received picture carrier frequency) to heterodyne with the picture carrier and produce an IF picture carrier of 45.75 MHz. Since the sound carrier is displaced 4.5 MHz from the picture carrier, the IF sound carrier has a frequency of 41.25 MHz. (The sound IF carrier becomes lower in frequency than the picture IF carrier because of the heterodyning process.)
The resulting IF signals are translated through appropriate television receiver circuitry and detected. In general practice, a discriminator or detector having a tuning or center frequency of 45.75 MHz, corresponding to the picture IF, is supplied with the IF signal and develops an error voltage dependent upon the polarity of the difference between the received picture IF carrier and the center frequency of the discriminator. In some situations, the magnitude of the error voltage is also a factor. The error voltage is fed to appropriate circuitry in the tuner for controlling the frequency of the oscillator based upon the polarity and/or magnitude thereof. The oscillator frequency changes, in response to the error voltage, to bring the received signal picture IF carrier frequency closer to the center frequency of the discriminator. Equilibrium is reached when the error voltage produced and the system gain operate on the oscillator to maintain tuning close to the discriminator center frequency, at which point the receiver should be properly tuned, i.e., to within its specified frequency tolerance.
As is well known, AFC systems may be designed to exhibit a variety of characteristics with specific characteristics appropriately emphasized according to need. For example, the main criterion for the AFC system may be the frequency range over which it can "pull in" a signal, which denotes the amount of oscillator frequency change available for tuning the receiver. The "holding" or "lock" range of the AFC system is also of importance and is a measure of the amount of the oscillator frequency drift (or received signal frequency change) which may be tolerated before noticeably distorting the receiver video display or audio accompaniment.
Needless to say, numerous types and variations of AFC circuits have been used over the years. In general, they have given acceptable performance and have proven a very useful feature on a television receiver. With the advent of color television and the NTSC signal with its color subcarrier, the requirements for AFC circuits become more stringent because slight detuning of the receiver may give rise to objectionable color shifts in the reproduced image.
The very recent trend to electronic type television tuning systems has added still another dimension to the frequency control problem. Such systems may not have an auxiliary control to enable the operator to manually adjust tuning to compensate for oscillator mistuning or signal conditions. Unlike their predecessor mechanical detent tuners, in which the nominal oscillator frequency is established by a tuning position--which also indicates the channel number of the signal tuned at that position--the electronic system may derive the channel number from the incoming signal, a preprogrammed memory or from the oscillator frequency. In the old system, the fine tuning control enabled reception of the proper television signal even though the nominal oscillator frequency substantially differed from its correct value. Generally, the AFC circuit was only made operative after tuning to the desired signal, for the sole purpose of maintaining that tuning. In many receivers with all-electronic tuning systems, fine tuning controls are available only at greatly increased cost and system complexity. Further in all-electronic tuning systems which derive the channel number of the received signal from either a memory or from the oscillator frequency, drift, either in the oscillator or received signal, may destroy the correlation between channel number and received signal.
A further problem arises because of the large number of television stations now in operation, both in the VHF and UHF bands. In the early days of television, frequency allocations were made with a view to avoidance of interference from adjacent channel signals. Now, however, in many parts of the country, it is not uncommon for receivers to be subject to very strong adjacent channel signals and consequent interference. Also with the growth of CATV and MATV, it is not unusual to have large numbers of adjacent channels because of their signal transposition to the VHF band. To add even more complexity, many MATV translated signals do not have the correct frequencies for the designated channels and consequently the picture carrier frequencies may deviate substantially. In these instances a true no-fine-tune receiver needs an AFC which is capable of differentiating between desired and non-desired signals--an enormously complicated systems problem.
Thus these problems associated with color television, electronic tuning, adjacent channel signals and non-standard signals with off-frequency carriers impose stringent demands on AFC systems. The art, in turn, has labored to improve the pull-in, hold-in and signal recognition capabilities of AFC systems. What the art has failed to do is to develop a new kind of AFC system which takes advantage of the fact that the picture and sound IF carriers are both present and spaced a predetermined distance apart. As will be seen, the AFC system of the invention utilizes both the IF sound and picture carriers for tuning the oscillator.
U.S. Pat. No. 3,459,887 dated Aug. 5, 1969 to R. F. Baker and assigned to Zenith Radio Corporation discloses an AFC system which utilizes the presence of the sound IF carrier to avoid an ambiguity in tuning and assist pull-in when the oscillator is tuned too high. The response attained with the Baker system is indicated by the solid line curve in FIG. 3A of the drawing and contains a so-called "negative hook" in the vicinity of the sound IF carrier frequency. This sharpens the response in this area and eliminates a condition where the AFC circuit might produce equal and opposite error voltages which would preclude moving the oscillator frequency into proper tuning. As will be seen, however, the sound IF carrier is only used in the event of serious mistuning in one direction and is not a contributing factor in maintaining proper tuning. It will be appreciated that serious mistuning with prior art AFC systems represents a comparatively small frequency differential. On the contrary, the wide range AFC system of the invention spans 4.5 MHz and has a response characteristic sharply bounded by frequencies corresponding to the IF sound and picture carrier frequencies. The system of the invention eliminates ambiguities in tuning and nullifies the effects of adjacent channel signal interference.